In recent years, an alloy particle comprising silver and copper is drawing an attention as a material used in an electrically conductive paste, an electrically conductive ink, an electrically conductive fine wiring, and the like; or as a material for a reducing catalyst of carbon monoxide and nitrogen oxides (NOx) as well as for a lead-free soldering. There is a possibility of controlling characteristics thereof by the ratio of silver to copper in a silver-copper alloy particle; and for example, a silver-copper alloy particle mainly made of silver, comprising silver which has superior specific resistance and oxidation resistance and copper which can suppress migration of silver, and a silver-copper alloy particle mainly made of copper useful as the material for wiring such as a magnet wire are drawing attention, too. Furthermore, antimicrobial properties of each of silver and copper are drawing attention, and in addition, many applications thereof including use in jewelry are expected; and thus, the silver-copper alloy is a widely wanted material in the industrial world. Migration occurs in many metals, and it is known that silver migrates faster; and it is said that migration thereof can be made slower by alloying it with other metals such as copper. However, the alloy of silver with copper is generally eutectic, so that in many cases the characteristics expected as the silver-copper alloy are not fully realized in suppressing the oxidation susceptibility of copper, suppressing migration of silver, and the like.
As to the producing methods of the silver-copper alloy particle, there are such methods as a liquid-phase reduction method, an atomizing method, and so forth, as described in Patent Document 1, Patent Document 2, and Patent Document 3. However, the silver-copper alloy obtained by any of these methods is a core-shell type or contains a eutectic; and therefore, there has been no disclosure as to the silver-copper alloy substantially not containing a eutectic and the producing method thereof. In Patent Document 1, the silver-core and the silver-copper-shell nanoparticle is mentioned, wherein the silver-copper alloy to constitute the shell is described from the elemental analysis in combination of the electron microscopic observation and the energy dispersive X-ray fluorescence measurement. However, because mapping of each of silver and copper in the shell part is not disclosed, and also for other reasons, there still remains the question as to whether or not silver and copper form the solid solution. In Patent Document 4, it is described that silver-covered copper powder obtained by covering the copper particle surface with silver was heat-treated at 150 to 600° C. under the non-oxidative atmosphere thereby dispersing silver to the copper particle to obtain the silver-dispersed copper powder. However, because the silver-dispersed copper powder is produced by dispersing silver metal from the copper particle surface, it is difficult to disperse silver to the central part of the copper particle; and thus, not only it is difficult to have the state not containing the eutectic in the entire particle thereof, but also the particle diameter thereof is too large to be used as a paste. Moreover, with regard to the analysis method of the silver-dispersed metal powder, there is a possibility that the copper single body might be present in the central part of the particle as it might also be the case that by heat treatment the metal silver that was present as the single body thereof on surface of the copper particle could not be confirmed merely by the surface observation (scanning electron microscope (SEM) observation). From these considerations, microscopically the above-mentioned silver-copper alloy cannot be regarded as the alloy, though macroscopically it may be regarded as the alloy.
In addition, there is a method such as for example in which a partial solid solution of the silver-copper alloy particle is obtained by rapidly cooling from the state that the metal silver and the metal copper are co-melted at high temperature; however, there has been no disclosure as to the silver-copper alloy having mainly the non-eutectic structure such as the solid solution. On top of this, the production thereof requires high energy so that this method automatically leads to problems such as high production cost.
In Patent Document 5, which is filed by the present applicant, the producing method of the silver-copper alloy particle is disclosed; however, analysis of the particle obtained by the producing method thereof shown by Example reveals that this particle is the silver-copper alloy particle formed of the eutectic or mixture of single bodies of silver and copper. Accordingly, there has been no disclosure as to the silver-copper alloy substantially not containing the eutectic body, especially as to the solid solution type silver-copper alloy.
As to the tin-silver-copper alloy, only the eutectic alloy thereof has been disclosed, as shown in Patent Document 6; and thus, there has been no disclosure as to the metal alloy mainly having the non-eutectic structure substantially not containing the eutectic.